Owing to the increasing scarcity of non-renewable energy reserves such as coal, petroleum and uranium, increased use is being made of alternative nondepletable energy sources, such as photovoltaic energy. Single crystal photovoltaic devices, especially crystalline silicon photovoltaic devices have been utilized for some time as sources of electrical power because they are inherently non-polluting, silent and consume no expendable natural resources in their operation. However, the utility of such devices has been limited by problems associated with the manufacture thereof. More particularly, single crystal materials (1) are difficult to produce in sizes substantially larger than several inches in diameter; (2) are thicker and heavier than their thin film counterparts; and (3) are expensive and time consuming to fabricate.
Recently, considerable effort has been expended to develop systems and processes for preparing thin film amorphous semiconductor alloy materials which encompass relatively large areas and which can be deposited so as to form p-type and n-type semiconductor alloy layers for the production therefrom of thin film electronic devices, particularly thin film p-n type and n-i-p type photovoltaic devices which are substantially operatively equivalent or superior to their crystalline counterparts. It should be noted at this point that the term "amorphous" as used herein, is defined to include alloys or materials exhibiting long range disorder, although said alloys or materials may exhibit short or intermediate range order or even contain crystalline inclusions. Also, as used herein, the term "microcrystalline" is defined as a unique class of said amorphous materials characterized by a volume fraction of crystalline inclusions, said volume fraction of inclusions being greater than a threshold value at which the onset of substantial changes in certain key parameters such as electrical conductivity, band gap and absorption constant occur. It is to be noted that pursuant to the foregoing definitions, the microcrystalline, p-doped, wide band gap, semiconductor alloy material referred to herein falls within the generic term "amorphous".
Amorphous thin film semiconductor alloys have gained acceptance for the fabrication of electronic devices such as photovoltaic cells, photoresponsive and photoconductive devices, transistors, diodes, integrated circuits, memory arrays and the like. This is because the amorphous thin film semiconductor alloys (1) can now be manufactured by relatively low cost continuous processes, (2) possess a wide range of controllable electrical, optical and structural properties and (3) can be deposited to cover relatively large areas. Among the semiconductor alloy materials exhibiting the greatest present commercial significance are amorphous silicon, germanium and silicon-germanium based alloys. Such alloys have been the subject of a continuing development effort on the part of the assignee of the instant invention, said alloys being investigated and utilized as possible candidates from which to fabricate a wide range of semiconductor, eletronic and photoresponsive devices.
The assignee of the present invention is recognized as the work leader in photovoltaic technology. Photovoltaic devices produced by said assignee have set world records for photoconversion efficiency and long term stablility under operating conditions (the efficiency and stability considerations will be discussed in great detail hereinbelow). Additionally, said assignee has developed commercial processes for the continuous roll-to-roll manufacture of large area photovoltaic devices. Such continuous processing systems are disclosed in the following U.S. Patents, disclosures of which are incorporated herein by reference: U.S. Pat. No. 4,400,409, for A Method Of Making P-Doped Silicon Films And Devices Made Therefrom; U.S. No. Pat. 4,410,588, for Continuous Amorphous Solar Cell Production Systems; and U.S. Pat. No. 4,438,723, for Multiple Chamber Deposition and Isolation System And Method. As disclosed in these patents a web of substrate material may be continuously advanced through a succession of operatively interconnected, environmentally protected deposition chambers, wherein each chamber is dedicated to the deposition of a specific layer of semiconductor alloy material onto the web or onto a previously deposited layer. In making a photovoltaic device, for instance, of n-i-p type configurations, the first chamber is dedicated for the deposition of a layer of an n-type semiconductor alloy material, the second chamber is dedicated for the deposition of a layer of substantially intrinsic amorphous semiconductor alloy material, and the third chamber is dedicated for the deposition of a layer of a p-type semiconductor alloy material. The layers of semiconductor alloy material thus deposited in the vacuum envelope of the deposition apparatus may be utilized to form photoresponsive devices, such as, but not limited to, photovoltaic devices which include one or more cascaded n-i-p type cells. By making multiple passes through the succession of deposition chambers, or by providing an additional array of deposition chambers, multiple stacked cells of various configurations may be obtained. Note, that as used herein the term "n-i-p type" will refer to any sequence of n and p or n, i and p semiconductor alloy layers operatively disposed and successively deposited to form a photoactive region wherein charge carriers are generated by the absorbtion of photons from incident radiation.
The concept of utlizing multiple stacked cells, to enhance photovoltaic device efficiency, was described at least as early as 1955 by E. D. Jackson in U.S. Pat. No. 2,949,498 issued Aug. 16, 1960. The multiple cell structures therein discussed were limited to the utilization of p-n junctions formed by single crystalline semiconductor devices. Essentially the concept employed different band gap devices to more efficiently collect various portions of the solar spectrum and to increase open circuit voltage (Voc). The tandem cell device (by definition) has two or more cells with the light directed serially through each cell. In the first cell, a large band gap material absorbs only the sort wavelength light, while in subsequent cells, smaller band gap materials absorb the longer wavelengths of light which pass through the first cell. By substantially matching the generated currents from each cell, the overall open circuit voltage is the sum of the open circuit voltage of each cell, while the short circuit current thereof remains substantially constant. Such tandem cell structures can be economically fabricated in large areas by employing thin film amorphous, semiconductor alloy materials (with or without crystalline inclusions), in accordance with the principles of the instant invention. It should be noted that Jackson employed crystalline semiconductor materials for the fabrication of his stacked cell structure; however, since it is virtually impossible to match lattice contents of differing crystalline materials, it is not possible to fabricate such crystalline tandem cell structures in a commercially feasible manner. In contrast thereto, and as the assignee of the instant invention has shown, such tandem cell structures are not only possible, but can be economically fabricated over large areas by employing the amorphous semiconductor alloy materials and the deposition techniques discussed and briefly described herein.
More particularly, the assignee of the instant invention is presently able to manufacture stacked, large area photovoltaic devices on a commercial basis by utilizing the previously referenced, continuous deposition, roll-to-roll processor. That processor is characterized by the assignee as a 1.5 megawatt capcity machine insofar as its annual output of photovoltaic devices is capable of producing 1.5 megawatts of electrical power. Said 1.5 megawatt processor, as presently configured, is adapted to produce tandem photovoltaic cells which comprise two stacked n-i-p type photovoltaic devices disposed optically and electrically in series upon a stainlees steel substrate. The processor currently includes six operatively interconnected, dedicated deposition chambers, each deposition chamber adapted to sequentially deposit one of the layers of semiconductor alloy material from which the tandem device is fabricated. The deposition chambers vary in length depending upon the thickness of the particular layer of semiconductor alloy material to be deposited therein.
More specifically, the thicknesses of individual layers of semiconductor alloy material vary from approximately 100 angstroms for the doped layers to approximately 3500 angstroms for the lowermost intrinsic layer. Since the processor operates by developing an r.f. plasma which is adapted to decompose the process gases and deposits a layer of semiconductor alloy material and the thickness of the deposited layer is directly dependent upon the residence time of the web of substrate material in the deposition chamber, the 3500 angstrom thick layer requires a deposition chamber of over six feet in length in order to provide an annual output of 1.5 megawatts of electrical power. The 1.5 megawatt processor also includes additional chambers for (1) the payoff and takeup of the web of substrate material, (2) the cleaning of the web of substrate material and (3) preventing interdiffusion of the gaseous contents of the adjacent deposition environments, said interdiffusion prevention preferably occuring in external gas gate boxes. With the addition of all of these chambers, the total length of the 1.5 megawatt processor comes to approximately 40 feet. Further, under operating conditions, the 1.5 megawatt processor utilizes only 10% of the process gas which is introduced thereinto. Accordingly, it must be appreciated that, while this 1.5 megawatt processor is the first apparatus capable of commercially fabricating photovoltaic devices; it is a complex, greatly elongated piece of machinery in which gas utilization has as yet to be optimized.
The assignee of the instant invention is now constructing a new and improved semiconductor processing machine for the production of significantly higher quantities of photovoltaic energy, about 25 megawatts of electrical power. it must be noted that in order to produce an annual output of 25 megawatts, the length of the machine must be increased so that this 25 megawatt processor will be at least an order of magnitude longer than the present 1.5 megawatt machine. Since not all of the reasons for this increased length are readily apparent, they will be enumerated in the following paragraphs.
A first reason for the elongation is that the new processor will be configured to fabricate tandem photovoltaic devices which comprise at least 3 and possibly 4 stacked cells; therefore the processor will require 9 to 12 dedicated deposition chambers instead of the six dedicated deposition chambers required by the present processor. As another factor in determining the length of the processor, and as mentioned previously, the length of each of the individual deposition chambers is dependent upon the thickness of each of the layers of semiconductor alloy material to be deposited thereon. The thickness of that material is, in turn, dependent upon, the rate of deposition of particular mixtures of precursor gases and the speed of the web of substrate material passing through that chamber of the processor. Consequently, if the rate of deposition of the precursor gas mixture remains constant (and Applicants find that increasing the rate of deposition of semiconductor alloy material tends to decrease the photovoltaic properties of that material), the web speed will also have to be kept constant and the deposition chambers in the 25 megawatt processor will have to be over sixteen times longer than in the 1.5 megawatt processor in order to deposit a sufficient quantity of semiconductor alloy material for fabricating photovoltaic devices which would provide an annual output of 25 megawatts of electrical power. Even assuming that the presently employed one foot wide web of substrate material were increased in size to a 2 foot width, a scaled-up version of the present processor which is designed to have a 25 megawatt capacity would still total approximately 400 feet in length. Even more significantly, in deposition apparatus of this size, the cathode utilized for the deposition of the thickest layer of semiconductor alloy material, i.e., the bottommost intrinsic layer of semiconductor alloy material of the tandem photovoltaic device, would have to be approximately 60 feet in length.
Clearly, a 400 foot long processor which requires the incorporation of a 60 foot long cathode presents many problems. The physical space required to house a machine greater than the length of 11/2 football fields presents problems in plant design, location and cost. Additionally, the mechanical design and operation of such a large, complex machine creates engineering problems related to the maintenance of the required optical, electrical and structural characteristics of the deposited semiconductor alloy material. The length and weight of the 500 foot span of substrate material, which continuously moves through the deposition apparatus, makes web handling and steering difficult, which, in turn, provides for numerous problems in maintaining substrate tracking, alignment and support. Likewise, maintenance of preselected vacuum conditions and deposition parameters within the 400 foot long vacuum envelope which the web of substrate material must traverse is, at best, quite difficult. Similarly, physical maintenance, i.e., disassembly, cleaning, etc. of the deposition apparatus becomes a nightmare.
Even more importantly (because it directly relates to the deposition of uniform, high quality semiconductor alloy material), the large areas covered by some of the deposition cathodes in such a scaled-up 25 megawatt processor creates problems of plasma uniformity and gas utilization within the cathode and deposition regions. Of the foregoing, plasma uniformity poses the most significant problem. Due to the large area plasma regions created by such large area cathodes, nonuniformities in the ionized precursor process gas mixtures are likely to arise. More specifically, varying compositions of the activated process gas mixture along the length of a large area cathode will give rise to irregular and nonhomogeneous plasma sub-regions, which irregularities and nonhomogeneties will result in the deposition of nonuniform, nonhomogeneous layers of semiconductor alloy material.
Gas utilization, yet another major problem encountered in the 25 megawatt processor, may be defined as the yield of semiconductor alloy material per unit of process gas introduced into the processor. Gas utilization is relatively poor (only about 10%) in the 1.5 megawatt processor and if the utilization of feedstock process gases is not significantly improved in the 25 megawatt processor, the cost of said process gases would become one of the largest factors in determining the ability of amorphous photovoltaic devices to economically compete with crystalline devices or nonrenewable energy sources for a share of the power consumption market.
It should be abundantly clear from the foregoing discussion that, as the 1.5 megawatt continuous photovoltaic device production machine is scaled up to higher throughput capacities, it becomes an economic necessity to substantially reduce the overll length thereof and substantially improve gas utilization. Such improvements would result in a substantial savings of time, floor space, the cost of building the machine and the operating cost for the production of photovoltaic devices therein.
The Assignee of the instant application has previously disclosed the concept of utilizing a non-horizontally disposed cathode plate in order to simultaneously deposit semiconductor alloy material in discrete plasma regions developed adjacent both of the opposed faces of that plate. This concept is described in U.S. Pat. No. 4,423,701 filed Mar. 29, 1982 of Prem Nath and David A. Gattuso entitled Glow Discharge Deposition Apparatus Including A Non-Horizontally Disposed Cathode, which patent is assigned to the assignee of the instant invention. Prior to the disclosure of said patent, only one-half (one face) of the potential surface area (two faces) of an r.f. powered cathode plate was utilized from which to develop a plasma, thereby limiting to one the number of substrates on which layers of amorphous semiconductor alloy material could be simultaneously deposited. The vertical orientation of the cathode plate, as described in said patent, provided the further advantage that deposition debris which is generated during the decomposition of the precursor gaseous mixture could not as readily come to rest on the vertically disposed surface of the substrate. Therefore, a continuous processor, utilizing such a generally vertically disposed cathode plate arrangement, would require less down time for dismantling, cleaning and reassembling. Finally, said above-referenced patent recognized the possibility of utilizing two webs of substrate material for the simultaneous and continuous deposition onto each of the webs of successive layers of semiconductor alloy material as said webs moved through the discrete plasma regions, developed on both faces of the cathode plates in each of the deposition chambers, in a generally linear path of travel.
However, while the deposition apparatus generally disclosed in U.S. Pat. No. 4,423,701 described a process of and apparatus for developing a plasma region adjacent each of the opposed faces of a generally vertically disposed cathode plate in order to continuously and simulataneously deposit layers of semiconductor alloy material onto each of two webs of substrate material as those webs passed through a plurality of interconnected deposition chambers, that process still failed to solve the one problem primarily at issue herein, i.e., the problem of reducing the length of the continuous processor so as to provide a commercially viable deposition process capable of depositing successive layers of semiconductor alloy material for fabricating triple or four (quad) cell tandem photovoltaic devices and having an annual capacity of up to 25 megawatts of electrical power. It is further desirable that this 25 megawattt capacity processor have the ability to increase the utilization of feedstock gases (vis-a-vis, single web systems) and to decrease down time (vis-a-vis, single web systems) due to the necessity of dismantling, cleaning and reassembling the cathode plates thereof caused by the accumulation of deposition debris.
These and other objects and advantages of the present invention will become clear from the Detailed Description of the Invention, the Drawings and the Claims which follow.